Method and system for forming patterns using charged particle beam lithography with variable pattern dosage

ABSTRACT

A method and system for fracturing or mask data preparation or optical proximity correction or proximity effect correction or mask process correction is disclosed in which a set of shaped beam shots is determined that is capable of forming a pattern on a surface, where the set of shots provides different dosages to different parts of the pattern, and where the dose margin from the set of shots is calculated. A method for forming patterns on a surface is also disclosed.

RELATED APPLICATIONS

This application: 1) is a continuation-in-part of U.S. patentapplication Ser. No. 13/037,263 filed on Feb. 28, 2011 entitled “MethodAnd System For Design Of A Surface To Be Manufactured Using ChargedParticle Beam Lithography”; 2) is related to U.S. patent applicationSer. No. 13/037,268 filed on Feb. 28, 2011 entitled “Method And SystemFor Design Of Enhanced Accuracy Patterns For Charged Particle BeamLithography”; and 3) is related to Fujimura, U.S. patent applicationSer. No. ______, entitled “Method and System For Forming Patterns UsingCharged Particle Beam Lithography With Overlapping Shots” (AttorneyDocket No. D2SiP032CIP1) filed on even date herewith, all of which arehereby incorporated by reference for all purposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto the design and manufacture of a surface which may be a reticle, awafer, or any other surface, using charged particle beam lithography.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit (I.C.). Other substratescould include flat panel displays, holographic masks or even otherreticles. While conventional optical lithography uses a light sourcehaving a wavelength of 193 nm, extreme ultraviolet (EUV) or X-raylithography are also considered types of optical lithography in thisapplication. The reticle or multiple reticles may contain a circuitpattern corresponding to an individual layer of the integrated circuit,and this pattern can be imaged onto a certain area on the substrate thathas been coated with a layer of radiation-sensitive material known asphotoresist or resist. Once the patterned layer is transferred the layermay undergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Eventually, a combination of multiplesof devices or integrated circuits will be present on the substrate.These integrated circuits may then be separated from one another bydicing or sawing and then may be mounted into individual packages. Inthe more general case, the patterns on the substrate may be used todefine artifacts such as display pixels, holograms, or magneticrecording heads. Conventional optical lithography writing machinestypically reduce the photomask pattern by a factor of four during theoptical lithographic process. Therefore, patterns formed on the reticleor mask must be four times larger than the size of the desired patternon the substrate or wafer.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, non-optical methods may be used to transfer apattern on a lithographic mask to a substrate such as a silicon wafer.Nanoimprint lithography (NIL) is an example of a non-optical lithographyprocess. In nanoimprint lithography, a lithographic mask pattern istransferred to a surface through contact of the lithography mask withthe surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nanoimprint lithography, or even reticles. Desiredpatterns of a layer are written directly on the surface, which in thiscase is also the substrate. Once the patterned layer is transferred thelayer may undergo various other processes such as etching,ion-implantation (doping), metallization, oxidation, and polishing.These processes are employed to finish an individual layer in thesubstrate. If several layers are required, then the whole process orvariations thereof will be repeated for each new layer. Some of thelayers may be written using optical lithography while others may bewritten using maskless direct write to fabricate the same substrate.Also, some patterns of a given layer may be written using opticallithography, and other patterns written using maskless direct write.Eventually, a combination of multiples of devices or integrated circuitswill be present on the substrate. These integrated circuits are thenseparated from one another by dicing or sawing and then mounted intoindividual packages. In the more general case, the patterns on thesurface may be used to define artifacts such as display pixels,holograms, or magnetic recording heads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane (i.e. of“manhattan” orientation), and 45 degree right triangles (i.e. triangleswith their three internal angles being 45 degrees, 45 degrees, and 90degrees) of certain minimum and maximum sizes. At predeterminedlocations, doses of electrons are shot into the resist with these simpleshapes. The total writing time for this type of system increases withthe number of shots. In character projection (CP), there is a stencil inthe system that has in it a variety of apertures or characters which maybe complex shapes such as rectilinear, arbitrary-angled linear,circular, nearly circular, annular, nearly annular, oval, nearly oval,partially circular, partially nearly circular, partially annular,partially nearly annular, partially nearly oval, or arbitrarycurvilinear shapes, and which may be a connected set of complex shapesor a group of disjointed sets of a connected set of complex shapes. Anelectron beam can be shot through a character on the stencil toefficiently produce more complex patterns on the reticle. In theory,such a system can be faster than a VSB system because it can shoot morecomplex shapes with each time-consuming shot. Thus, an E-shaped patternshot with a VSB system takes four shots, but the same E-shaped patterncan be shot with one shot with a character projection system. Note thatVSB systems can be thought of as a special (simple) case of characterprojection, where the characters are just simple characters, usuallyrectangles or 45-45-90 degree triangles. It is also possible topartially expose a character. This can be done by, for instance,blocking part of the particle beam. For example, the E-shaped patterndescribed above can be partially exposed as an F-shaped pattern or anI-shaped pattern, where different parts of the beam are cut off by anaperture. This is the same mechanism as how various sized rectangles canbe shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection.

As indicated, in lithography the lithographic mask or reticle comprisesgeometric patterns corresponding to the circuit components to beintegrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce the original circuit design on the substrate by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in conventional optical lithography. As the criticaldimensions of the circuit pattern become smaller and approach theresolution value of the exposure tool, the accurate transcription of thephysical design to the actual circuit pattern developed on the resistlayer becomes difficult. To further the use of optical lithography totransfer patterns having features that are smaller than the lightwavelength used in the optical lithography process, a process known asoptical proximity correction (OPC) has been developed. OPC alters thephysical design to compensate for distortions caused by effects such asoptical diffraction and the optical interaction of features withproximate features. OPC includes all resolution enhancement technologiesperformed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on an interior orexterior corner of a pattern to sharpen the corner in the finaltransferred image. It is often the case that the precision demanded ofthe surface manufacturing process for SRAFs is less than the precisiondemanded for patterns that are intended to print on the substrate, oftenreferred to as main features. Serifs are a part of a main feature. Asthe limits of optical lithography are being extended far into thesub-wavelength regime, the OPC features must be made more and morecomplex in order to compensate for even more subtle interactions andeffects. As imaging systems are pushed closer to their limits, theability to produce reticles with sufficiently fine OPC features becomescritical. Although adding serifs or other OPC features to a mask patternis advantageous, it also substantially increases the total feature countin the mask pattern. For example, adding a serif to each of the cornersof a square using conventional techniques adds eight more rectangles toa mask or reticle pattern. Adding OPC features is a very laborious task,requires costly computation time, and results in more expensivereticles. Not only are OPC patterns complex, but since optical proximityeffects are long range compared to minimum line and space dimensions,the correct OPC patterns in a given location depend significantly onwhat other geometry is in the neighborhood. Thus, for instance, a lineend will have different size serifs depending on what is near it on thereticle. This is even though the objective might be to produce exactlythe same shape on the wafer. These slight but critical variations areimportant and have prevented others from being able to form reticlepatterns. It is conventional to discuss the OPC-decorated patterns to bewritten on a reticle in terms of designed features, that is featuresthat reflect the design before OPC decoration, and OPC features, whereOPC features might include serifs, jogs, and SRAF. To quantify what ismeant by slight variations, a typical slight variation in OPC decorationfrom neighborhood to neighborhood might be 5% to 80% of a designedfeature size. Note that for clarity, variations in the design of the OPCare what is being referenced. Manufacturing variations such as cornerrounding will also be present in the actual surface patterns. When theseOPC variations produce substantially the same patterns on the wafer,what is meant is that the geometry on the wafer is targeted to be thesame within a specified error, which depends on the details of thefunction that that geometry is designed to perform, e.g., a transistoror a wire. Nevertheless, typical specifications are in the 2%-50% of adesigned feature range. There are numerous manufacturing factors thatalso cause variations, but the OPC component of that overall error isoften in the range listed. OPC shapes such as sub-resolution assistfeatures are subject to various design rules, such as a rule based onthe size of the smallest feature that can be transferred to the waferusing optical lithography. Other design rules may come from the maskmanufacturing process or, if a character projection charged particlebeam writing system is used to form the pattern on a reticle, from thestencil manufacturing process. It should also be noted that the accuracyrequirement of the SRAF features on the mask may be lower than theaccuracy requirements for the designed features on the mask. As processnodes continue to shrink, the size of the smallest SRAFs on a photomaskalso shrinks For example, at the 20 nm logic process node, 40 nm to 60nm SRAFs are needed on the mask for the highest precision layers.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the substrate as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since these ideal ILT curvilinear patterns are difficult andexpensive to form on a reticle using conventional techniques,rectilinear approximations or rectilinearizations of the curvilinearpatterns may be used. The rectilinear approximations decrease accuracy,however, compared to the ideal ILT curvilinear patterns. Additionally,if the rectilinear approximations are produced from the ideal ILTcurvilinear patterns, the overall calculation time is increased comparedto ideal ILT curvilinear patterns. In this disclosure ILT, OPC, sourcemask optimization (SMO), and computational lithography are terms thatare used interchangeably.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. The most commonly used system is the variable shaped beam(VSB), where, as described above, doses of electrons with simple shapessuch as manhattan rectangles and 45-degree right triangles expose aresist-coated reticle surface. In conventional mask writing, the dosesor shots of electrons are conventionally designed to avoid overlapwherever possible, so as to greatly simplify calculation of how theresist on the reticle will register the pattern. Similarly, the set ofshots is designed so as to completely cover the pattern area that is tobe formed on the reticle. U.S. Pat. No. 7,754,401, owned by the assigneeof the present patent application and incorporated by reference for allpurposes, discloses a method of mask writing in which intentional shotoverlap for writing patterns is used. When overlapping shots are used,charged particle beam simulation can be used to determine the patternthat the resist on the reticle will register. Use of overlapping shotsmay allow patterns to be written with reduced shot count. U.S. Pat. No.7,754,401 also discloses use of dose modulation, where the assigneddosages of shots vary with respect to the dosages of other shots. Theterm model-based fracturing is used to describe the process ofdetermining shots using the techniques of U.S. Pat. No. 7,754,401.

Reticle writing for the most advanced technology nodes typicallyinvolves multiple passes of charged particle beam writing, a processcalled multi-pass exposure, whereby the given shape on the reticle iswritten and overwritten. Typically, two to four passes are used to writea reticle to average out precision errors in the charged particle beamwriter, allowing the creation of more accurate photomasks. Alsotypically, the list of shots, including the dosages, is the same forevery pass. In one variation of multi-pass exposure, the lists of shotsmay vary among exposure passes, but the union of the shots in anyexposure pass covers the same area. Multi-pass writing can reduceover-heating of the resist coating the surface. Multi-pass writing alsoaverages out random errors of the charged particle beam writer.Multi-pass writing using different shot lists for different exposurepasses can also reduce the effects of certain systemic errors in thewriting process.

In EUV lithography, OPC features are generally not required. Therefore,the complexity of the pattern to be manufactured on the reticle is lessthan with conventional 193 nm wavelength optical lithography, and shotcount reduction is correspondingly less important. In EUV, however, maskaccuracy requirements are very high because the patterns on the mask,which are typically 4× the size of the patterns on the wafer, aresufficiently small that they are challenging to form precisely usingcharged particle beam technology such as electron beam.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation or opticalproximity correction or proximity effect correction or mask processcorrection is disclosed in which a set of shaped beam shots isdetermined that is capable of forming a pattern on a surface, where theset of shots provides different dosages to different parts of thepattern, and where the dose margin from the set of shots is calculated.

A method for forming patterns on a surface is also disclosed, in which aset of shaped beam shots is determined that is capable of forming apattern on a surface, where the set of shots provides different dosagesto different parts of the pattern, and where the dose margin from theset of shots is calculated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a character projection particle beamsystem;

FIG. 2A illustrates an example of a single charged particle beam shotand a cross-sectional dosage graph of the shot;

FIG. 2B illustrates an example of a pair of proximate shots and across-sectional dosage graph of the shot pair;

FIG. 2C illustrates an example of a pattern formed on a resist-coatedsurface from the pair of FIG. 2B shots;

FIG. 3A illustrates an example of a polygonal pattern;

FIG. 3B illustrates an example of a conventional fracturing of thepolygonal pattern of FIG. 3A;

FIG. 3C illustrates an example of an alternate fracturing of thepolygonal pattern of FIG. 3A;

FIG. 4A illustrates an example of a shot outline from a rectangularshot;

FIG. 4B illustrates an example of a longitudinal dosage curve for theshot of FIG. 4A using a normal shot dosage;

FIG. 4C illustrates an example of a longitudinal dosage curve similar toFIG. 4B, with long-range effects included;

FIG. 4D illustrates an example of a longitudinal dosage curve for theshot of FIG. 4A using a higher than normal shot dosage;

FIG. 4E illustrates an example of a longitudinal dosage curve similar toFIG. 4D, with long-range effects included;

FIG. 4F illustrates an example of a longitudinal dosage curve similar toFIG. 4E, but with a higher background dosage level;

FIG. 5A illustrates an example of a circular pattern to be formed on asurface;

FIG. 5B illustrates an example of outlines of nine shots which can formthe pattern of FIG. 5A;

FIG. 6A illustrates a square pattern to be formed on a surface;

FIG. 6B illustrates a single-shot method of forming the pattern of FIG.6A on a surface;

FIG. 6C illustrates an example of a method of forming the pattern ofFIG. 6A on a surface by another embodiment of the current invention;

FIG. 6D illustrates an example of a method of forming the pattern ofFIG. 6A on a surface by yet another embodiment of the current invention;

FIG. 7 illustrates a conceptual flow diagram of how to prepare asurface, such as a reticle, for use in fabricating a substrate such asan integrated circuit on a silicon wafer using optical lithography; and

FIG. 8 illustrates a conceptual flow diagram of how to prepare a surfacefor use in fabricating a substrate such as an integrated circuit on asilicon wafer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes a method for fracturing patterns intoshots for a charged particle beam writer, where overlapping shots aregenerated to improve the accuracy and/or the dose margin of the patternwritten to a surface. The dose margin improvement reduces dimensionalchanges in the written pattern which are associated with processvariations.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a conventional lithography system100, such as a charged particle beam writer system, in this case anelectron beam writer system, that employs character projection tomanufacture a surface 130. The electron beam writer system 100 has anelectron beam source 112 that projects an electron beam 114 toward anaperture plate 116. The plate 116 has an aperture 118 formed thereinwhich allows the electron beam 114 to pass. Once the electron beam 114passes through the aperture 118 it is directed or deflected by a systemof lenses (not shown) as electron beam 120 toward another rectangularaperture plate or stencil mask 122. The stencil 122 has formed therein anumber of openings or apertures 124 that define various types ofcharacters 126, which may be complex characters. Each character 126formed in the stencil 122 may be used to form a pattern 148 on a surface130 of a substrate 132, such as a silicon wafer, a reticle or othersubstrate. In partial exposure, partial projection, partial characterprojection, or variable character projection, electron beam 120 may bepositioned so as to strike or illuminate only a portion of one of thecharacters 126, thereby forming a pattern 148 that is a subset ofcharacter 126. For each character 126 that is smaller than the size ofthe electron beam 120 defined by aperture 118, a blanking area 136,containing no aperture, is designed to be adjacent to the character 126,so as to prevent the electron beam 120 from illuminating an unwantedcharacter on stencil 122. An electron beam 134 emerges from one of thecharacters 126 and passes through an electromagnetic or electrostaticreduction lens 138 which reduces the size of the pattern from thecharacter 126. In commonly available charged particle beam writersystems, the reduction factor is between 10 and 60. The reduced electronbeam 140 emerges from the reduction lens 138, and is directed by aseries of deflectors 142 onto the surface 130 as the pattern 148, whichis depicted as being in the shape of the letter “H” corresponding tocharacter 126A. The pattern 148 is reduced in size compared to thecharacter 126A because of the reduction lens 138. The pattern 148 isdrawn by using one shot of the electron beam system 100. This reducesthe overall writing time to complete the pattern 148 as compared tousing a variable shape beam (VSB) projection system or method. Althoughone aperture 118 is shown being formed in the plate 116, it is possiblethat there may be more than one aperture in the plate 116. Although twoplates 116 and 122 are shown in this example, there may be only oneplate or more than two plates, each plate comprising one or moreapertures. Stencil mask 122 also contains apertures for VSB shots, suchas rectangular aperture 125.

In conventional charged particle beam writer systems the reduction lens138 is calibrated to provide a fixed reduction factor. The reductionlens 138 and/or the deflectors 142 also focus the beam on the plane ofthe surface 130. The size of the surface 130 may be significantly largerthan the maximum beam deflection capability of the deflection plates142. Because of this, patterns are normally written on the surface in aseries of stripes. Each stripe contains a plurality of sub-fields, wherea sub-field is within the beam deflection capability of the deflectionplates 142. The electron beam writer system 100 contains a positioningmechanism 150 to allow positioning the substrate 132 for each of thestripes and sub-fields. In one variation of the conventional chargedparticle beam writer system, the substrate 132 is held stationary whilea sub-field is exposed, after which the positioning mechanism 150 movesthe substrate 132 to the next sub-field position. In another variationof the conventional charged particle beam writer system, the substrate132 moves continuously during the writing process. In this variationinvolving continuous movement, in addition to deflection plates 142,there may be another set of deflection plates (not shown) to move thebeam at the same speed and direction as the substrate 132 is moved. Inone embodiment the substrate 132 may be a reticle. In this embodiment,the reticle, after being exposed with the pattern, undergoes variousmanufacturing steps through which it becomes a lithographic mask orphotomask. The mask may then be used in an optical lithography machineto project an image of the reticle pattern 148, generally reduced insize, onto a silicon wafer to produce an integrated circuit. Moregenerally, the mask is used in another device or machine to transfer thepattern 148 on to a substrate. In another embodiment the substrate 132may be a silicon wafer.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 130 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 100 and with thesurface 130, which normally comprises a resist coating on the substrate132. These effects include forward scattering, Coulomb effect, andresist diffusion. Beam blur, also called β_(f), is a term used toinclude all of these short-range effects. The most modern electron beamwriter systems can achieve an effective beam blur radius or β_(f) in therange of 20 nm to 30 nm. Forward scattering may constitute one quarterto one half of the total beam blur. Modern electron beam writer systemscontain numerous mechanisms to reduce each of the constituent pieces ofbeam blur to a minimum. Since some components of beam blur are afunction of the calibration level of a particle beam writer, the β_(f)of two particle beam writers of the same design may differ. Thediffusion characteristics of resists may also vary. Variation of β_(f)based on shot size or shot dose can be simulated and systemicallyaccounted for. But there are other effects that cannot or are notaccounted for, and they appear as random variation.

The shot dosage of a charged particle beam writer such as an electronbeam writer system is a function of the intensity of the beam source 112and the exposure time for each shot. Typically the beam intensityremains fixed, and the exposure time is varied to obtain variable shotdosages. The exposure time may be varied to compensate for variouslong-range effects such as backscatter and fogging in a process calledproximity effect correction (PEC). Electron beam writer systems usuallyallow setting an overall dosage, called a base dosage, which affects allshots in an exposure pass. Some electron beam writer systems performdosage compensation calculations within the electron beam writer systemitself, and do not allow the dosage of each shot to be assignedindividually as part of the input shot list, the input shots thereforehaving unassigned shot dosages. In such electron beam writer systems allshots have the base dosage, before PEC. Other electron beam writersystems do allow dosage assignment on a shot-by-shot basis. In electronbeam writer systems that allow shot-by-shot dosage assignment, thenumber of available dosage levels may be 64 to 4096 or more, or theremay be a relatively few available dosage levels, such as 3 to 8 levels.Some embodiments of the current invention are targeted for use withcharged particle beam writing systems which allow assignment of one of arelatively few dosage levels.

The mechanisms within electron beam writers have a relatively coarseresolution for calculations. As such, mid-range corrections such as maybe required for EUV masks in the range of 2 μm cannot be computedaccurately by current electron beam writers.

FIGS. 2A-B illustrate how energy is registered on a resist-coatedsurface from one or more charged particle beam shots. In FIG. 2Arectangular pattern 202 illustrates a shot outline, which is a patternthat will be produced on a resist-coated surface from a shot which isnot proximate to other shots. The corners of pattern 202 are rounded dueto beam blur. In dosage graph 210, dosage curve 212 illustrates thecross-sectional dosage along a line 204 through shot outline 202. Line214 denotes the resist threshold, which is the dosage above which theresist will register a pattern. As can be seen from dosage graph 210,dosage curve 212 is above the resist threshold between the X-coordinates“a” and “b”. Coordinate “a” corresponds to dashed line 216, whichdenotes the left-most extent of the shot outline 202. Similarly,coordinate “b” corresponds to dashed line 218, which denotes theright-most extent of the shot outline 202. The shot dosage for the shotin the example of FIG. 2A is a normal dosage, as marked on dosage graph210. In conventional mask writing methodology, the normal dosage is setso that a relatively large rectangular shot will register a pattern ofthe desired size on the resist-coated surface, in the absence oflong-range effects. The normal dosage therefore depends on the value ofthe resist threshold 214.

FIG. 2B illustrates the shot outlines of two particle beam shots, andthe corresponding dosage curve. Shot outline 222 and shot outline 224result from two proximate particle beam shots. In dosage graph 220,dosage curve 230 illustrates the dosage along the line 226 through shotoutlines 222 and 224. As shown in dosage curve 230, the dosageregistered by the resist along line 226 is the combination, such as thesum, of the dosages from two particle beam shots, represented by shotoutline 222 and shot outline 224. As can be seen, dosage curve 230 isabove the threshold 214 from X-coordinate “a” to X-coordinate “d”. Thisindicates that the resist will register the two shots as a single shape,extending from coordinate “a” to coordinate “d”. FIG. 2C illustrates apattern 252 that the two shots from the example of FIG. 2B may form. Thevariable width of pattern 252 is the result of the gap between shotoutline 222 and shot outline 224, and illustrates that a gap between theshots 222 and 224 causes dosage to drop below threshold near the cornersof the shot outlines closest to the gap.

When using non-overlapping shots using a single exposure pass,conventionally all shots are assigned a normal dosage before PEC dosageadjustment. A charged particle beam writer which does not supportshot-by-shot dosage assignment can therefore be used by setting the basedosage to a normal dosage. If multiple exposure passes are used withsuch a charged particle beam writer, the base dosage is conventionallyset according to the following equation:

base dosage=normal dosage/#of exposure passes

FIGS. 3A-C illustrate two known methods of fracturing a polygonalpattern. FIG. 3A illustrates a polygonal pattern 302 that is desired tobe formed on a surface. FIG. 3B illustrates a conventional method offorming this pattern using non-overlapping or disjoint shots. Shotoutline 310, shot outline 312 and shot outline 314, which are markedwith X's for clarity, are mutually disjoint. Additionally, the threeshots associated with these shot outlines all use a desired normaldosage, before proximity effect correction. An advantage of using theconventional method as shown in FIG. 3B is that the response of theresist can be easily predicted. Also, the shots of FIG. 3B can beexposed using a charged particle beam system which does not allow dosageassignment on a shot-by-shot basis, by setting the base dosage of thecharged particle beam writer to the normal dosage. FIG. 3C illustratesan alternate method of forming the pattern 302 on a resist-coatedsurface using overlapping shots, which is disclosed in U.S. Pat. No.7,754,401. In FIG. 3C the constraint that shot outlines cannot overlaphas been eliminated, and shot 320 and shot 322 do overlap. In theexample of FIG. 3C, allowing shot outlines to overlap enables formingthe pattern 302 in only two shots, compared to the three shots of FIG.3B. In FIG. 3C, however the response of the resist to the overlappingshots is not as easily predicted as in FIG. 3B. In particular, theinterior corners 324, 326, 328 and 330 may register as excessivelyrounded because of the large dosage received by overlapping region 332,shown by horizontal line shading. Charged particle beam simulation maybe used to determine the pattern registered by the resist. In oneembodiment disclosed in U.S. Pat. No. 8,062,813, which is owned by theassignee of the present patent application and incorporated by referencefor all purposes, charged particle beam simulation may be used tocalculate the dosage for each grid location in a two-dimensional (X andY) grid, creating a grid of calculated dosages called a dosage map. Theresults of charged particle beam simulation may indicate use ofnon-normal dosages for shot 320 and shot 322. Additionally, in FIG. 3Cthe overlapping of shots in region 332 increases the area dosage—thedosage in the area proximate to pattern 302—beyond what it would bewithout shot overlap, thereby increasing backscatter. While the overlapof two individual shots will not increase the area dosage significantly,this technique will increase backscatter if used throughout a design.

In exposing, for example, a repeated pattern on a surface using chargedparticle beam lithography, the size of each pattern instance, asmeasured on the final manufactured surface, will be slightly different,due to manufacturing variations. The amount of the size variation is anessential manufacturing optimization criterion. In mask masking today, aroot mean square (RMS) variation of no more than 1 nm (1 sigma) may bedesired. More size variation translates to more variation in circuitperformance, leading to higher design margins being required, making itincreasingly difficult to design faster, lower-power integratedcircuits. This variation is referred to as critical dimension (CD)variation. A low CD variation is desirable, and indicates thatmanufacturing variations will produce relatively small size variationson the final manufactured surface. In the smaller scale, the effects ofa high CD variation may be observed as line edge roughness (LER). LER iscaused by each part of a line edge being slightly differentlymanufactured, leading to some waviness in a line that is intended tohave a straight edge. CD variation is inversely related to the slope ofthe dosage curve at the resist threshold, which is called edge slope.Therefore, edge slope, or dose margin, is a critical optimization factorfor particle beam writing of surfaces.

FIG. 4A illustrates an example of an outline of a rectangular shot 402.FIG. 4B illustrates an example of a dosage graph 410 illustrating thedosage along the line 404 through shot outline 402 with a normal shotdosage, with no backscatter, such as would occur if shot 402 was theonly shot within the range of the backscattering effect, which, as anexample, may be 10 microns. Other long-range effects are also assumed tocontribute nothing to the background exposure of FIG. 4B, leading to azero background exposure level. The total dosage delivered to the resistis illustrated on the y-axis, and is 100% of a normal dosage. Because ofthe zero background exposure, the total dosage and the shot dosage arethe same. Dosage graph 410 also illustrates the resist threshold 414.The CD variation of the shape represented by dosage graph 410 in thex-direction is inversely related to the slope of the dosage curve 412 atx-coordinates “a” and “b” where it intersects the resist threshold.

The FIG. 4B condition of zero background exposure is not reflective ofactual designs. Actual designs will typically have many other shotswithin the backscattering distance of shot 402. FIG. 4C illustrates anexample of a dosage graph 420 of a shot with a normal total dosage withnon-zero background exposure 428. In this example, a background exposureof 20% of a normal dosage is shown. In dosage graph 420, dosage curve422 illustrates the cross-sectional dosage of a shot similar to shot402. The CD variation of curve 422 is worse than the CD variation ofcurve 412, as indicated by the lower edge slope where curve 422intersects resist threshold 424 at points “a” and “b”, due to thebackground exposure caused by backscatter.

One method of increasing the slope of the dosage curve at the resistthreshold is to increase the shot dosage. FIG. 4D illustrates an exampleof a dosage graph 430 with a dosage curve 432 which illustrates a totaldosage of 150% of normal dosage, with no background exposure. With nobackground exposure, the shot dosage equals the total dosage. Threshold434 in FIG. 4D is unchanged from threshold 414 in FIG. 4B. Increasingshot dosage increases the size of a pattern registered by the resist.Therefore, to maintain the size of the resist pattern, illustrated asthe intersection points of dosage curve 432 with threshold 434, the shotsize used for dosage graph 430 is somewhat smaller than shot 402. As canbe seen, the slope of dosage curve 432 is higher where it intersectsthreshold 434 than is the slope of dosage curve 412 where it intersectsthreshold 414, indicating a lower, improved, CD variation for thehigher-dosage shot of FIG. 4D than for the normal dosage shot of FIG.4B.

Like dosage graph 410, however, the zero background exposure conditionof dosage graph 430 is not reflective of actual designs. FIG. 4Eillustrates an example of a dosage graph 440 with the shot dosageadjusted to achieve a total dosage on the resist of 150% of normaldosage with a 20% background exposure, such as would occur if the dosageof only one shot was increased to achieve total dosage of 150% of anormal dosage, and dosage of other shots remained at 100% of normaldosage. The threshold 444 is the same as in FIGS. 4B-4D. The backgroundexposure is illustrated as line 448. As can be seen, the slopes ofdosage curve 442 at x-coordinates “a” and “b” are less than the slopesof dosage curve 432 at x-coordinates “a” and “b” because of the presenceof backscatter. Comparing graphs 420 and 440 for the effect of shotdosage, the slope of dosage curve 442 at x-coordinates “a” and “b” ishigher than the slope of dosage curve 422 at the same x-coordinates,indicating that improved edge slope can be obtained for a single shot byincreasing dosage, if dosages of other shots remain the same.

FIG. 4F illustrates an example of a dosage graph 450, illustrating thecase where the dosages of all shots have been increased to 150% ofnormal dose. Two background dosage levels are shown on dosage graph 450:a 30% background dose 459, such as may be produced if all shots use 150%of normal dosage, and a 20% background dose 458 shown for comparison,since 20% is the background dosage in the dosage graph 440. Dosage curve452 is based on the 30% background dose 459. As can be seen, the edgeslope of dosage curve 452 at x-coordinates “a” and “b” is less than thatof dosage curve 442 at the same points.

In summary, FIGS. 4A-F illustrate that higher-than-normal dosage can beused selectively to lower CD variation for isolated shapes. Increasingdosage has two undesirable effects, however. First, an increase in doseis achieved in modern charged particle beam writers by lengtheningexposure time. Thus, an increase in dose increases the writing time,which increases cost. Second, as illustrated in FIGS. 4E-F, if manyshots within the backscatter range of each other use an increaseddosage, the increase in backscatter reduces the edge slope of all shots,thereby worsening CD variation for all shots of a certain assigneddosage. The only way for any given shot to avert this problem is toincrease dosage and shoot a smaller size. However, doing this increasesthe backscatter even more. This cycle causes all shots to be at a higherdose, making write times even worse. Therefore, it is better to increasedose only for shots that define the edge.

Edge slope or dose margin is an issue only at pattern edges. If, forexample, the normal dosage is 2× the resist threshold, so as to providea good edge slope, the interior areas of patterns can have a dosagelower than normal dosage, so long as dosage in all interior areasremains above the resist threshold, after accounting for some margin formanufacturing variation. In the present disclosure, two methods ofreducing the dosage of interior areas of a pattern are disclosed:

-   -   If assigned shot dosages are available, use lower-than-normal        shot dosages.    -   Insert gaps between shots in the interior of patterns. Although        the shot outlines may show gaps, if the dosage within the gap        area is everywhere above the resist threshold, with margin        provided for manufacturing variation, no gap will be registered        by the resist. Either or both of these techniques will reduce        the area dosage, thus reducing the background dosage caused by        backscatter. Edge slope at the pattern edges will therefore be        increased, thereby improving CD variation.

Optimization techniques may be used to determine the lowest dosage thatcan be achieved in interior portions of the pattern. In someembodiments, these optimization techniques will include calculating theresist response to the set of shots, such as with using particle beamsimulation, so as to determine that the set of shots forms the desiredpattern, perhaps within a predetermined tolerance. Note that whencreating shots for a charged particle beam writer which supports onlyunassigned dosage shots, gaps can be used in interior areas of thepattern to reduce area dosage. By simulating, particularly with the“corner cases” of the manufacturing tolerance, designs with lower dosesor gaps can be pre-determined to shoot the desired shapes safely withreduced write time and improved edge slope.

FIG. 5A illustrates an example of a circular pattern 502 that is to beformed on a surface. FIG. 5B illustrates an example of how the pattern502 may be formed with a set of nine VSB shots with assigned shotdosages. FIG. 5B illustrates the shot outlines of each of the nineshots. In FIG. 5B, overlapping shots 512, 514, 516, 518, 520, 522, 524and 526 may be assigned a relatively higher set of dosages, or in someembodiments all assigned a normal dosage, to maintain a good edge slope,since each of these shots defines the perimeter of the pattern on thesurface. Shot 530, however, may have an assigned dosage less than shots512, 514, 516, 518, 520, 522, 524 and 526, such as 0.7× a normal dosage,since shot 530 does not define an edge of the pattern. That is,different dosages are provided to different parts of the pattern. Theshot sizes will be carefully chosen so as not to have any portion of theinterior of shape 502 fall below the resist threshold, perhaps with somemargin for manufacturing variation. Shot 530 may also be sized so that agap exists between the outline of shot 530 and the outline of each ofthe adjacent shots, as illustrated in FIG. 5B. When a gap is present,the union of outlines of shots in the set of shots does not cover thedesired pattern. Particle beam simulation may be used to determine anoptimal size for the gap so that dosage may be reduced without causing agap to be registered by the resist. The use of lower-than-normal dosagefor shot 530, when applied to a large number of such shots within thebackscatter range of each other, will reduce the backscatter andfogging, contributing to improved edge slope, compared to exposing shot530 and the large number of other shots within the backscatter rangewith a normal dosage.

The solution described above with FIG. 5B may be implemented even usinga charged particle beam system that does not allow dosage assignment forindividual shots. In one embodiment of the present invention, a smallnumber of dosages may be selected, for example two dosages such as 1.0×normal and 0.7× normal, and shots for each of these two dosages may beseparated and exposed in two separate exposures passes, where the basedosage for one exposure pass is 1.0× normal and the base dosage for theother exposure pass is 0.7× normal. In the example of FIG. 5B, shots512, 514, 516, 518, 520, 522, 524 and 526 may be assigned to a firstexposure pass which uses a base dosage of 1.0× normal dosage before PECcorrection. Shot 530 may be assigned to a second exposure pass whichuses a base dosage of 0.7× normal dosage before PEC correction.

Overlapping shots may be used to create resist dosages greater than 100%of normal, even with charged particle beam writers which do not supportdosage assignment for individual shots. In FIG. 5B, for example outlinesfor shots 514 and 512, shots 526 and 524, shots 520 and 522, and shots518 and 516 may be designed to overlap, creating regions ofhigher-than-normal dosage in the periphery. The higher energy that iscast from these regions can “fill in” the gap between shot outline 530and the peripheral shots, making it possible to decrease the size ofshot 530.

FIGS. 6A-D illustrate the use of overlapping shots with square patterns,such as are commonly used for contact and via patterns in integratedcircuit design. FIG. 6A illustrates an example of a desired pattern 602to be formed on a reticle. FIG. 6B illustrates a single VSB shot 612which may be used to form pattern 602 conventionally. Use of single shot612 may cause edge slope to be undesirably low, however. FIG. 6Cillustrates an example of an embodiment of the present invention. FIG.6C may be shot using five VSB shots, including shot 632, which iscross-hatched, and four additional shots 634 around the perimeter areasof the original pattern 602. Also, a CP character may be designed toexpose the pattern illustrated by the four rectangles 634 in a single CPshot, allowing FIG. 6C to be exposed in one VSB shot 632 and one CP shotfor all shapes 634. The use of the perimeter CP shot or VSB shots canincrease the edge slope of the entire perimeter of the transferredpattern by increasing peak dosage near the perimeter, compared to theinterior area, thus also providing different dosages to different partsof the pattern. The small perimeter CP shot or VSB shots do not increasethe area dosage as much as if a higher dosage was used for shot 612,reducing the backscatter compared to if a higher dosage shot 612 wasused alone.

FIG. 6D illustrates an example of another embodiment of the presentinvention. Nine regions are illustrated in FIG. 6D: a) a large region642, b) four side regions 644, and c) four corner regions 648. As can beseen, all regions 644 and 648 overlap region 642. These regions may beexposed by any of the following methods:

-   -   Nine separate VSB shots, including one for region 642, four        shots for the four regions 644, and four shots for the four        corner regions 648.    -   Five VSB shots. Region 642 is exposed by one shot. For the        remaining four

VSB shots, each shot includes the union of one side region 644 and twocorner regions 648 adjacent to the side regions. This provides a higherdosage at the corners than along the side perimeters. The additionalpeak exposure near the corner may provide improved accuracy and/or edgeslope.

-   -   One VSB shot for region 642 and two CP shots—one shot each of        two CP characters. One CP character may be designed, for example        to include the four side regions 644 and a second CP character        may be designed to include the four corner regions 648. This        solution allows independent dosage control of the corner regions        and non-corner side regions.        The method using one VSB shot with two CP shots should require        less exposure time than either the nine-shot VSB or the        five-shot VSB methods. Additionally, the size of shot 642 may be        modified to be smaller than the desired pattern 602.

The solution described above with FIG. 6C may be implemented even usinga charged particle beam system that does not allow dosage assignment forindividual shots. In one embodiment of the present invention, a smallnumber of dosages may be selected, for example two dosages such as 1.0×normal and 0.6× normal, and shots for each of these two dosages may beseparated and exposed in two separate exposures passes, where the basedosage for one exposure pass is 1.0× normal and the base dosage for theother exposure pass is 0.6× normal. In the example of FIG. 6C, shot 632may be assigned to a first exposure pass which uses a base dosage of1.0× normal dosage before PEC correction. The four shots 634 may beassigned to a second exposure pass which uses a base dosage of 0.6×normal dosage before PEC correction. Thus, overlapping shots can createpattern dosages greater than 100% of normal, even with charged particlebeam writers which do not support dosage assignment for individualshots.

In one embodiment of the invention, gaps between normal-dosage ornear-normal-dosage shots may be filled or partially filled withlow-dosage shots, such as shots having less than 50% of normal dosage.

The calculations described or referred to in this invention may beaccomplished in various ways. Generally, calculations may beaccomplished by in-process, pre-process or post-process methods.In-process calculation involves performing a calculation at the timewhen its results are needed. Pre-process calculation involvespre-calculating and then storing results for later retrieval during asubsequent processing step, and may improve processing performance,particularly for calculations that may be repeated many times.Calculations may also be deferred from a processing step and then donein a later post-processing step. An example of pre-process calculationis pre-calculating PEC dosage adjustments for various values ofbackscatter. Another example of pre-process calculation is a shot group,which is a pre-calculation of dosage pattern information for one or moreshots associated with a given input pattern or set of input patterncharacteristics. The shot group and the associated input pattern may besaved in a library of pre-calculated shot groups, so that the set ofshots comprising the shot group can be quickly generated for additionalinstances of the input pattern, without pattern re-calculation. In someembodiments, the pre-calculation may comprise simulation of the dosagepattern that the shot group will produce on a resist-coated surface. Inother embodiments, the shot group may be determined without simulation,such as by using correct-by-construction techniques. In someembodiments, the pre-calculated shot groups may be stored in the shotgroup library in the form of a list of shots. In other embodiments, thepre-calculated shot groups may be stored in the form of computer codethat can generate shots for a specific type or types of input patterns.In yet other embodiments, a plurality of pre-calculated shot groups maybe stored in the form of a table, where entries in the table correspondto various input patterns or input pattern characteristics such aspattern width, and where each table entry provides either a list ofshots in the shot group, or information for how to generate theappropriate set of shots. Additionally, different shot groups may bestored in different forms in the shot group library. In someembodiments, the dosage pattern which a given shot group can produce mayalso be stored in the shot group library. In one embodiment, the dosagepattern may be stored as a two-dimensional (X and Y) dosage map called aglyph.

FIG. 7 is an exemplary conceptual flow diagram 750 of how to prepare areticle for use in fabricating a surface such as an integrated circuiton a silicon wafer. In a first step 752, a physical design, such as aphysical design of an integrated circuit, is designed. This can includedetermining the logic gates, transistors, metal layers, and other itemsthat are required to be found in a physical design such as that in anintegrated circuit. The physical design may be rectilinear, partiallycurvilinear, or completely curvilinear. Next, in a step 754, opticalproximity correction is determined. In an embodiment of this disclosurethis can include taking as input a library of pre-calculated shot groupsfrom a shot group library 788. This can also alternatively, or inaddition, include taking as input a library of pre-designed characters780 including complex characters that are to be available on a stencil784 in a step 768. In an embodiment of this disclosure, an OPC step 754may also include simultaneous optimization of shot count or write times,and may also include a fracturing operation, a shot placement operation,a dose assignment operation, or may also include a shot sequenceoptimization operation or dose margin optimization, or other mask datapreparation operations, with some or all of these operations beingsimultaneous or combined in a single step. The OPC step 754 may createpartially or completely curvilinear patterns. The output of the OPC step754 is a mask design 756.

Mask process correction (MPC) 758 may optionally be performed on themask design 756. MPC modifies the pattern to be written to the mask soas to compensate for non-linear effects, such as effects associated withpatterns smaller than about 100 nm in conventional optical lithographicmasks. MPC may also be used to compensate for non-linear effectsaffecting EUV masks. If MPC 758 is performed, its output becomes theinput for mask data preparation (MDP) step 760.

In a step 760, a mask data preparation operation which may include afracturing operation, a shot placement operation, a dose assignmentoperation, or a shot sequence optimization may take place. MDP may useas input the mask design 756 or the results of MPC 758. In someembodiments of the present invention, MPC may be performed as part of afracturing or other MDP operation. Other corrections may also beperformed as part of fracturing or other MDP operation, the possiblecorrections including: forward scattering, resist diffusion, Coulombeffect, etching, backward scattering, fogging, loading, resist charging,and EUV midrange scattering. The result of MDP step 760 is a shot list762, either for one or for multiple exposure passes in mask writing step768. Either OPC step 754 or MDP step 760, or a separate program 786 caninclude pre-calculating one or more shot groups that may be used for agiven input pattern, and storing this information in a shot grouplibrary 788. Combining OPC and any or all of the various operations ofmask data preparation in one step is contemplated in this disclosure.Mask data preparation step 760, which may include a fracturingoperation, may also comprise a pattern matching operation to matchpre-calculated shot groups to create a mask image 770 that matchesclosely to the mask design 756. Mask data preparation 760 may alsoinclude calculating the dose margin, and may also include optimizing thedose margin. In some embodiments, optimization may include varying shotdosages to produce a higher peak dosage near perimeters of generatedpatterns than in the interior of the generated patterns. In otherembodiments, generated shots may have gaps between the shot outlines ofnearest neighboring shots, so that area dosage is decreased, but wherethe gaps are sufficiently small that they will not be registered by theresist in the subsequently-produced mask image 770. In anotherembodiment, mask data preparation 760 may include optimization bychanging the size of the gaps. In another embodiment, mask datapreparation 760 may include revising the initially-determined set ofshots if the calculated dose margin is below a pre-determined targetdose margin, and recalculating the dose margin with the revised set ofshots. Mask data preparation 760 may also comprise inputting patterns tobe formed on a surface with the patterns being slightly different,selecting a set of characters to be used to form the number of patterns,the set of characters fitting on a stencil mask, the set of characterspossibly including both complex and VSB characters, and the set ofcharacters based on varying character dose or varying character positionor applying partial exposure of a character within the set of charactersor dragging a character to reduce the shot count or total write time. Aset of slightly different patterns on the surface may be designed toproduce substantially the same pattern on a substrate. Also, the set ofcharacters may be selected from a predetermined set of characters. Inone embodiment of this disclosure, a set of characters available on astencil in the step 780 that may be selected quickly during the maskwriting step 768 may be prepared for a specific mask design. In thatembodiment, once the mask data preparation step 760 is completed, astencil is prepared in the step 784. In another embodiment of thisdisclosure, a stencil is prepared in the step 784 prior to orsimultaneous with the MDP step 760 and may be independent of theparticular mask design. In this embodiment, the characters available inthe step 780 and the stencil layout are designed in step 782 to outputgenerically for many potential mask designs 756 to incorporate patternsthat are likely to be output by a particular OPC program 754 or aparticular MDP program 760 or particular types of designs thatcharacterizes the physical design 752 such as memories, flash memories,system on chip designs, or particular process technology being designedto in physical design 752, or a particular cell library used in physicaldesign 752, or any other common characteristics that may form differentsets of slightly different patterns in mask design 756. The stencil caninclude a set of characters, such as a limited number of characters thatwas determined in the step 760.

In step 764 proximity effect correction (PEC) refinement may beperformed on shot list 762 to create a final shot list 766 with adjusteddosages. The final shot list 766 is used to generate a surface in a maskwriting step 768, which uses a charged particle beam writer such as anelectron beam writer system. In some embodiments, PEC refinement 764 maybe performed by the charged particle beam writer. Mask writing step 768may use stencil 784 containing both VSB apertures and a plurality ofcomplex characters, or may use a stencil comprising only VSB apertures.Mask writing step 768 may comprise a single exposure pass or multipleexposure passes. The electron beam writer system projects a beam ofelectrons through the stencil onto a surface to form a mask imagecomprising patterns on a surface, as shown in a step 770. The completedsurface may then be used in an optical lithography machine, which isshown in a step 772. Finally, in a step 774, a substrate such as asilicon wafer is produced.

As has been previously described, in step 780 characters may be providedto the OPC step 754 or the MDP step 760. The step 780 also providescharacters to a character and stencil design step 782 or to a shot grouppre-calculation step 786. The character and stencil design step 782provides input to the stencil step 784 and to the characters step 780.The shot group pre-calculation step 786 provides information to the shotgroup library 788. Also, the shot group pre-calculation step 786 may useas input the physical design 752 or the mask design 756, and maypre-calculate one or more shot groups, which are stored in a shot grouplibrary 788.

Referring now to FIG. 8, another exemplary conceptual flow diagram 800of how to prepare a surface which is directly written on a substratesuch as a silicon wafer is shown. In a first step 802, a physicaldesign, such as a physical design of an integrated circuit is designed.This may be an ideal pattern that the designer wants transferred onto asubstrate. Next, in a step 804, various data preparation (DP) steps areperformed to prepare input data to a substrate writing device. Step 804may include fracturing of the patterns into a set of VSB and/or complexCP shots, where some of the shots may overlap each other. Othercorrections may also be performed as part of fracturing or other DPoperations, the possible corrections including: forward scattering,resist diffusion, Coulomb effect, etching, backward scattering, fogging,loading, and resist charging. Either DP step 804 or a separate program822 can include pre-calculating one or more shot groups that may be usedfor a given input pattern, and storing this information in a shot grouplibrary 824. The step 804 may also comprise pattern matching to matchpre-calculated shot groups to create a wafer image 814 that matchesclosely to the physical design created in the step 802. Iterations ofpattern matching, dose assignment, and equivalence checking may also beperformed. In one embodiment, there may be a single iteration where acorrect-by-construction “deterministic” calculation is performed. Datapreparation 804 may include calculating the dose margin, and may alsoinclude optimizing the dose margin. In some embodiments optimization mayinclude varying shot dosages to produce a higher peak dosage nearperimeters of the generated patterns than in the interior of thegenerated patterns. In other embodiments, generated shots may have gapsbetween nearest neighboring shots, so that area dosage is decreased, butwhere the gaps are sufficiently small that they will not be registeredby the resist in the subsequently-produced wafer image 814. In anotherembodiment, step 804 may include optimization by changing the size ofthe gaps. In another embodiment, data preparation 804 may includerevising the initially-determined set of shots if the calculated dosemargin is below a pre-determined target dose margin, and recalculatingthe dose margin with the revised set of shots. The output of step 804 isshot list 806.

In step 808 proximity effect correction (PEC) may be performed on shotlist 806 to create a final shot list 810 with adjusted dosages. Thefinal shot list 810 is used to create a pattern on a surface such as animage on a wafer 814 in a wafer writing step 812 which uses a chargedparticle beam writer such as an electron beam writer system. In someembodiments, PEC refinement 808 may be performed by the charged particlebeam writer. Wafer writing step 812 may use stencil 808 containing bothVSB apertures and a plurality of complex characters, or may use astencil comprising only VSB apertures. In wafer writing step 812, theelectron beam writer system projects a beam of electrons through thestencil onto a surface to form an image 814 comprising patterns on thesurface. Wafer writing step 812 may comprise a single exposure pass ormultiple exposure passes.

As has been previously described, in step 818 characters may be providedto DP step 804. Step 818 also provides characters to a character andstencil design step 820 or to a shot group pre-calculation step 822. Thecharacter and stencil design step 820 provides input to the stencil step808 and to the characters step 818. The shot group pre-calculation step822 provides information to the shot group library 824. Also, the shotgroup pre-calculation step 822 may use as input the physical design 802and may pre-calculate one or more shot groups, which are stored in ashot group library 824.

The step 812 may include repeated application as required for each layerof processing, potentially with some processed using the methodsdescribed in association with FIG. 7, and others processed using themethods outlined above with respect to FIG. 8, or others produced usingany other wafer writing method to produce integrated circuits on thesilicon wafer.

The fracturing, MDP, OPC, MPC and PEC flows described in this disclosuremay be implemented using general-purpose computers with appropriatecomputer software as computation devices. Due to the large amount ofcalculations required, multiple computers or processor cores may also beused in parallel. In one embodiment, the computations may be subdividedinto a plurality of 2-dimensional geometric regions for one or morecomputation-intensive steps in the flow, to support parallel processing.In another embodiment, a special-purpose hardware device, either usedsingly or in multiples, may be used to perform the computations of oneor more steps with greater speed than using general-purpose computers orprocessor cores. In one embodiment, the special-purpose hardware devicemay be a graphics processing unit (GPU). In another embodiment, theoptimization and simulation processes described in this disclosure mayinclude iterative processes of revising and recalculating possiblesolutions, so as to minimize either the total number of shots, or thetotal charged particle beam writing time, or some other parameter. Inyet another embodiment, an initial set of shots may be determined in acorrect-by-construction method, so that no shot modifications arerequired.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, mask data preparation, and proximity effectcorrection may be practiced by those of ordinary skill in the art,without departing from the spirit and scope of the present subjectmatter, which is more particularly set forth in the appended claims.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tobe limiting. Steps can be added to, taken from or modified from thesteps in this specification without deviating from the scope of theinvention. In general, any flowcharts presented are only intended toindicate one possible sequence of basic operations to achieve afunction, and many variations are possible. Thus, it is intended thatthe present subject matter covers such modifications and variations ascome within the scope of the appended claims and their equivalents.

1. A method for fracturing or mask data preparation or proximity effectcorrection or optical proximity correction or mask process correctioncomprising the step of: determining a plurality of shaped beam chargedparticle beam shots for an exposure pass, wherein the plurality ofshaped beam shots, when used in a charged particle beam writer, producesa dosage on a resist-coated surface, wherein the dosage on theresist-coated surface forms a pattern on the resist-coated surface,wherein the pattern on the surface comprises a pattern perimeter,wherein the resist comprises a resist threshold, wherein the pluralityof shaped beam shots provides different dosages to different parts ofthe pattern, wherein the step of determining comprises calculating adose margin from the plurality of shaped beam shots, wherein the dosemargin is a slope of the resist dosage, at the resist threshold, withrespect to a linear dimension perpendicular to the pattern perimeter,and wherein the step of determining is performed using one or morecomputing hardware processors.
 2. The method of claim 1 wherein the dosemargin is optimized.
 3. The method of claim 2 wherein the plurality ofshaped beam shots produces a higher resist dosage peak near the patternperimeter than in an interior area of the pattern on the surface.
 4. Themethod of claim 1 wherein the calculating comprises charged particlebeam simulation.
 5. The method of claim 4 wherein the charged particlebeam simulation includes at least one of a group consisting of forwardscattering, backward scattering, resist diffusion, Coulomb effect,etching, fogging, loading and resist charging.
 6. The method of claim 1,further comprising the step of revising the plurality of shaped beamshots and recalculating the dose margin if the dose margin is lower thana pre-determined target dose margin.
 7. The method of claim 1 whereineach shot in the plurality of shaped beam shots comprises an assigneddosage, and wherein the assigned dosages of at least two shots in theplurality of shaped beam shots differ from each other before dosagecorrection for long-range effects.
 8. The method of claim 1 wherein eachshot in the plurality of shaped beam shots is a variable shaped beam(VSB) shot.
 9. The method of claim 1 wherein the surface comprises areticle to be used in an optical lithographic process to manufacture asubstrate.
 10. A method for manufacturing a surface using chargedparticle beam lithography, the method comprising the steps of:determining a plurality of shaped beam shots for a plurality of exposurepasses; and forming a pattern on the surface, wherein the surface hasbeen coated with a resist, wherein a charged particle beam writerproduces a dosage on the resist-coated surface using the plurality ofshaped beam shots, wherein the dosage on the resist-coated surface formsthe pattern, wherein the pattern on the surface comprises a patternperimeter, wherein the resist comprises a resist threshold, wherein theplurality of shaped beam shots provides different dosages to differentparts of the pattern, wherein the step of determining comprisescalculating a dose margin from the plurality of shaped beam shots, andwherein the dose margin is a slope of the resist dosage, at the resistthreshold, with respect to a linear dimension perpendicular to thepattern perimeter.
 11. The method of claim 10 wherein the dose margin isoptimized.
 12. The method of claim 11 wherein the plurality of shapedbeam shots produces a higher resist dosage peak near the patternperimeter on the surface than in an interior area of the pattern on thesurface.
 13. The method of claim 10 wherein the calculating comprisescharged particle beam simulation.
 14. The method of claim 13 wherein thecharged particle beam simulation includes at least one of a groupconsisting of forward scattering, backward scattering, resist diffusion,Coulomb effect, etching, fogging, loading and resist charging.
 15. Themethod of claim 10, further comprising the step of revising theplurality of shaped beam shots and recalculating the dose margin if thedose margin is lower than a pre-determined target dose margin.
 16. Themethod of claim 10 wherein each shot in the plurality of shaped beamshots comprises an assigned dosage, and wherein the assigned dosages ofat least two shots in the plurality of shaped beam shots differ beforedosage correction for long-range effects.
 17. The method of claim 16,further comprising the step of performing dose correction for long-rangeeffects, wherein the assigned dosages of at least two shots in theplurality of shaped beam shots differ from each other before the dosecorrection.
 18. The method of claim 10 wherein the surface comprises areticle to be used in an optical lithographic process to manufacture asubstrate.
 19. A method for manufacturing an integrated circuit using anoptical lithographic process, the optical lithographic process using areticle manufactured with charged particle beam lithography, the methodcomprising the steps of: determining a plurality of shaped beam shotsfor an exposure pass; and forming a pattern on the reticle, wherein thereticle has been coated with a resist, wherein a charged particle beamwriter produces a dosage on the resist-coated reticle using theplurality of shaped beam shots, wherein the dosage on the resist-coatedreticle forms the pattern, wherein the pattern on the reticle comprisesa pattern perimeter, wherein the resist comprises a resist threshold,wherein the plurality of shaped beam shots provides different dosages todifferent parts of the pattern, and wherein the step of determiningcomprises calculating a dose margin from the plurality of shaped beamshots, and wherein the dose margin is a slope of the resist dosage, atthe resist threshold, with respect to a linear dimension perpendicularto the pattern perimeter.
 20. The method of claim 19 wherein thecalculating comprises charged particle beam simulation.
 21. The methodof claim 20 wherein the charged particle beam simulation includes atleast one of a group consisting of forward scattering, backwardscattering, resist diffusion, Coulomb effect, etching, fogging, loadingand resist charging.
 22. A system for fracturing or mask datapreparation or proximity effect correction or optical proximitycorrection or mask process correction comprising: a device configured todetermine a plurality of shaped beam charged particle beam shots for anexposure pass, wherein the plurality of shaped beam shots, when used ina charged particle beam writer, produces a dosage on a resist-coatedsurface, wherein the dosage on the resist-coated surface forms a patternon the surface, wherein the pattern on the surface comprises a patternperimeter, wherein the resist comprises a resist threshold, wherein theplurality of shaped beam shots provides different dosages to differentparts of the pattern, and wherein the device configured to determinecalculates a dose margin from the plurality of shaped beam shots, andwherein the dose margin is a slope of the resist dosage, at the resistthreshold, with respect to a linear dimension perpendicular to thepattern perimeter.
 23. The system of claim 22 wherein the dose margin isoptimized.
 24. The system of claim 23 wherein the plurality of shapedbeam shots produces a higher resist dosage peak near the patternperimeter than in an interior area of the pattern on the surface. 25.The system of claim 22 wherein each shot in the plurality of shaped beamshots is a variable shaped beam (VSB) shot.